FIELD OF THE INVENTION
This invention relates to timekeeping and more particularly to a novel and highly effective timekeeping method and apparatus for solving a serious problem due to arise on Aug. 22, 1999. On that day, if no corrective action is taken in the meantime, the timekeeping signals provided by the most advanced navigation system in the world will become ambiguous.
Navigation, map-making and surveying have been revolutionized by the global positioning system, or GPS, provided by the U.S. Government and to a lesser extent by GLONASS, a satellite navigation system provided by the Russian Government. A European satellite navigation system is planned but not yet operational. GPS and other satellite navigation systems enable a determination of one's position and velocity speedily, accurately, automatically, and inexpensively.
The principles of satellite navigation are well known and will not be described in detail here. Those interested in the details can refer to a number of reference works, including GPS Theory and Practice, by B. Hofmann-Wellenhof, H. Lichtenegger, and J. Collins, Springer-Verlag, Vienna and New York, third edition, 1994.
In broad outline, satellite navigation works as follows (for convenience, GPS is summarized, but the principles apply to satellite navigation systems generally): Since Jan. 6, 1980, as many as two dozen GPS satellites have been in various earth orbits at a height of about 20,000 kilometers (see FIG. 8, which represents four such satellites S, a ground-based receiver R, and Earth E). Each satellite carries an atomic clock accurate to one second in several million years. The satellites, while of course always moving, are distributed around the earth so that typically seven or more are well above the horizon as seen from any point on or near the earth. The satellites transmit line-of-sight signals, so that, absent obstruction by mountains, buildings, etc., one can expect always to be able to receive signals from about seven satellites, regardless of one's location (on or near the earth). For a "complete" fix (time, longitude, latitude and elevation relative to sea level), a receiver, which can be in a car, on a plane, aboard a ship, hand-held, etc., must receive signals from at least four satellites. For a fix that dispenses, for example, with a readout of elevation, reception of signals from three satellites suffices. For a determination of time, a signal from one satellite suffices (if the receiver location is accurately known, the time signal carries full GPS accuracy; if the receiver location is not known, the time error is at most 21 ms).
Each GPS signal is encoded to indicate the time of transmission from the satellite and has several portions, including a coarse acquisition or CA code, a precise or P code, a portion representing a count of weeks since the inauguration of the GPS system on Jan. 1, 1980, and a data portion giving the satellite's status and orbital parameters.
Time is encoded on the CA code modulo 1 millisecond (1 ms) and on the P code modulo 1 week. All GPS receivers can receive and use the CA code, but the P code is encrypted and requires for its use decryption software available only to the U.S. military. Use of the P code gives a move precise fix, but GPS receivers without the decryption software can use the data portion of the signal, which gives the satellite's status and orbital parameters, to approximate closely the week boundaries. Thus all users of the GPS system can obtain and use a signal from each satellite sufficient to enable software in the receiver to calculate a fix rapidly and accurately.
However, the week count is encoded using only ten bits and is thus capable of a maximum count of 2.sup.10 =1024. In other words, 1024 weeks after the inauguration of the system on Jan. 6, 1980--i.e. on Aug. 22, 1999--, the week count will cycle from 1111111111 to 0000000000. As matters now stand, there will be nothing in the GPS data to tell explicitly that the year is in fact 1999 and not 1980.
If no corrective action is taken, this will cause problems with both navigation and timekeeping. The navigation problem arises from the need to relate dated satellite orbit information to the almanacs and ephemerides that inform the receiver of the satellites' positions. Here the difficulty is simply to relate the data and computations for the old week 1023 to the new week 1024, which is encoded not as "week 1024" but as "week 0". It is a simple computational matter to interpret the differences between week numbers as being in the interval, for example, -512 through +511, rather than the interval 0 through 1023, and thus avoid completely any disruption to navigation during the rollover.
The timekeeping problem is more difficult. Several solutions have been suggested. One is to design the system not to tolerate any date before it was manufactured, but rather to add the necessary multiple of 1024 weeks. That is satisfactory only for an additional 19.6 years; beyond that, the required multiple is ambiguous, and the rollover problem will arise again early in the year 2019.
Another possibility is to save the current date in nonvolatile memory. This converts the 20-year life into a more acceptable 20-year shelf life. However, it introduces a vulnerability to a mistaken date. For example, if the system is programmed never to allow time to run backward, erroneously decodes a date, places it in the future as instructed, and then decodes the true date, it will place the true date in the future from the false date and always show a net advance of 19.6 years. Variations on the date algorithm that allow backward time changes can recover the correct date after an error, but not reliably.
Another solution is to include a separate clock in each of the (myriad) receiving systems. This requires also adding to each receiver an uninterrupted source such as a battery to power the clock, since the clock must always be maintained, even during periods of nonuse. Although a clock in the receiver need not have high accuracy, a requirement for a local clock in each receiver adds to the cost, size and weight and is not a good solution to the rollover problem.
In principle, the rollover problem can be neatly solved by changing the GPS data format to encode the week count using more bits. That, however, would require a major revamping of the GPS system--a project that, because of the risk of confusing existing receivers, has not been, and perhaps should not be, undertaken.
What is needed is a solution that is sure to work for rather more than twenty years, using no external real-time clocks, nonvolatile memories, or user inputs.